1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to bonding semiconductors wafers to one another.
2. Related Art
Microelectromechanical systems (MEMS) are widely used in a variety of sensing applications. For example, a MEMS piezoresistive pressure sensor may be implemented on a semiconductor die to generate electrical signals indicative of the amount of pressure exerted on the semiconductor die (or a portion thereof). Some MEMS devices such as gyroscopes or resonators operate in a vacuum-sealed cavity. The stronger the vacuum level, the lower the power required to operate the MEMS device. With the continual demand to reduce power consumption, it is desirable to form vacuum levels that are as strong as possible.
Strong vacuum levels are more easily achieved and maintained if residual absorbed gases from the semiconductor wafers are removed before the vacuum seals are formed. Although the wafers can be baked to remove the residual absorbed gases before forming the vacuum seal, time required to bake the wafers increases costs and decreases wafer throughput. Alternatively, a material referred to as a “getter” can be used in the cavity to absorb the residual absorbed gases even after the cavity is vacuum sealed, but the getter material adds cost and complexity to wafer processing.
When baking is used to removed residual absorbed gases prior to vacuum bonding, a bottom wafer typically experiences poor thermal contact with a lower heater in the bonding equipment while the top wafer receives only radiant heating. Inability to effectively increase the temperature of the wafers increases the amount of time the wafers must be baked. Additionally, any difference in temperature between the top and bottom wafers can form mechanical stress on the bonded wafers. The stress can result in bowed wafers, which can present problems during subsequent processing and packaging, and decrease the yield.